The present invention relates to a method for forming rounded corners in the trench, and more particularly to a method of forming rounded corners in the trench of trench-type Metal-Oxide Semiconductor (trench-type MOS) devices or other integrated circuit elements.
Nowadays, trench-type MOS devices are widely used in the semiconductor industry. FIGS. 1(a) and 1(b) are diagrams illustrating part of a conventional fabrication method of the trench-type MOS devices. As shown in FIG. 1(a), in the manufacturing process of trench-type Metal-Oxide Semiconductor Field Effect Transistor (MOSFET), a semiconductor substrate 10 is firstly provided, and a pad oxide layer 11, a silicon nitride layer 12 and an oxide layer 13 are formed sequentially on the semiconductor substrate 10. Then, the oxide layer 13, the silicon nitride layer 12, the pad oxide layer 11 and the semiconductor substrate 10 are removed partially to form at least one trench 14 on the semiconductor substrate 10 with a conventional process, for example, the photolithography or etching process.
Because of the plasma etching process used for forming the trench 14, some lattice defects or non-planar surfaces are usually formed simultaneously on the sidewalls of the trench 14. As a result, in order to solve this problem, a sacrifice oxide layer (not shown) is first formed on the trench 14 and then removed later.
Then, as shown in FIG. 1(b), an oxide layer is formed on the oxide layer 13 and in the trench 14 to be a gate oxide layer or dielectric layer 15. After that, some processes, such as the formation of a bottom oxide layer (not shown) at the bottom of the trench 14 and/or the deposition of a polysilicon layer into the trench 14, will be further performed to complete the manufacturing process of the trench-type Metal-Oxide Semiconductor Field Effect Transistor (MOSFET).
As seen in FIG. 1(a), after the trench 14 is formed by the plasma etching process, the sidewalls of the trench 14 usually have a vertical profile, and the bottom corner of the trench forms nearly a right angle. Therefore, during the subsequent oxidation processes, a gate oxide layer or a dielectric layer 15 would be formed along the entire profile of the trench 14. After the subsequent oxidation processes, a polysilicon layer may be deposited in the trench 14 which is formed by first removing the oxide layer 13, the silicon nitride layer 12, and the pad oxide layer, with the top corners and the bottom corners of the trench 14 each nearly at a right angle. As a result, the lattices of the polysilicon near the top corners of the trench 14 would squeeze each other to apply corner stresses on the top corners of the trench 14. Moreover, the gate oxide layer or the dielectric layer 15 near the bottom corners of the trench 14 also would be formed with non-uniform thickness.
FIGS. 2(a) and 2(b) are scanning electron microscope (SEM) pictures of the top corners and bottom corners of the trench through the steps of the conventional manufacturing process. As illustrated in FIG. 2(a), the top corners of the trench 14 each have the profile of substantially a right angle. Thus, the lattices near the top corners of the trench 14 would squeeze each other. It leads to corner stresses in the trench 14. Therefore, point discharge would occur when a trench type MOSFET is operated. In addition, as shown in FIG. 2(b), the bottom corners of the trench 14 are not rounded, but are each at substantially a right angle. The thickness of the gate oxide layer or the dielectric layer 15 will be non-uniform when the gate oxide layer or the dielectric layer 15 is formed in the trench 14. This could cause serious current leakage when the trench type MOSFET is operated.